The selective deposition and removal of dielectric layers on and from portions of a substrate which includes a plurality of electronic components some of which are desired to be covered by the dielectric layer and some of which are not, presents great difficulties when the nature of dielectric material, the underlying layer, and the associated electronic components do not permit using the usual masking and etching techniques for the selective application and removal of the dielectric.
As a specific example of one such application where traditional techniques are not easy to use is in the manufacture of large scale radiation detection panels for use in medical radiography, such as described in U.S. Pat. No. 5,381,014 issued to L. Jeromin et al and assigned to the assignee of the present application, or to U.S. patent application Ser. No. 08/987,485 filed Dec. 9, 1997 entitled "Image capture element", also assigned to the assignee of the present invention. The contents of both are incorporated herein.
As disclosed in these two references, in the manufacture of a radiographic imaging panel such as disclosed therein, there is required to deposit a dielectric layer over the selenium photodetector layer which is substantially coextensive therewith. Such dielectric layer does not extend over portions of the panel along the edges thereof where other electronic components will eventually be connected to leads leading to the access lines and switching transistors associated with the individual radiation detectors in the imaging panel.
In the specific application discussed above, the specifications for the dielectric material, such as dielectric strength and physical properties, typically require a material which is hard to cut and remove along desired lines following its application. Typical such dielectric materials are inter alia, polypropylene, polyethylene terephthalate and parylene. The first two are provided in preformed sheet material which is applied and adhered onto the selenium surface. Because the material is very thin, it is difficult to apply without wrinkles. Parylene on the other hand may be deposited through vacuum deposition at low temperature on the selenium layer. Such process is well known, it is easy to control and yields a smooth coating over the panel surface. Parylene forms a tough layer which even though remains somewhat flexible after polymerization, is even harder to cut with a mechanical instrument than polyethylene terephthalate.
When parylene is used, prior to the parylene-coating step, masking tape has been used to mask the areas, such as the wirebonding pads, where coating is not desired. The resulting parylene and tape thereunder are then manually cut from the rest of the parylene layer with a razor blade. Because the dimensions of the electronic components are extremely small, such cutting is done under a microscope to assure that only the desired areas are removed. After cutting, the portion slated for removal is peeled away with the tape.
This delicate and tedious task can take a substantial amount of time and must be performed by a skilled individual. For example, a medical radiographic application panel has X and Y dimensions of 14 and 17 inches respectively, resulting in a 62 inch perimeter. Typically such panel will have a plurality of electronic components attached along all four sides which will be connected to contact pads on leads extending from under the dielectric layer. (See for instance FIG. 1). These contact pads must not be covered with the dielectric material. Applying a tape along the perimeter to cover the pads prior to the deposition of the dielectric layer and depositing a layer of parylene dielectric material thereover is simple. However, the subsequent cutting of the parylene layer along the masked edge under the microscope with a razor blade may take up to 1-1/2 hours. Furthermore, the mechanical cutting process exerts a compression force upon the layers of parylene, occasionally causing de-lamination at various interfaces.
There is still, therefore, need to provide a precise, preferably non-contacting process for selectively removing portions of a dielectric coating such as parylene from selected areas of a substrate more quickly and with less risk of damaging the remaining coating, than by a manual cutting operation.